Special counsel Robert S. Mueller III is investigating the finances and business dealings of Jared Kushner, President Trump’s son-in-law and adviser, as part of the investigation into Russia’s interference in the 2016 presidential election, according to U.S. officials familiar with the matter.

FBI agents and federal prosecutors have also been examining the financial dealings of other Trump associates, including former national security adviser Michael Flynn, former campaign chairman Paul Manafort and Carter Page, who was listed as a foreign-policy adviser for the campaign.

The Washington Post previously reported that investigators were scrutinizing meetings that Kushner held with Russians in December — first with Russian ambassador Sergey Kislyak, and then with Sergey Gorkov, the head of a state-owned Russian development bank. At the time of that report, it was not clear that the FBI was investigating Kushner’s business dealings.

The officials who described the financial focus of the investigation spoke on the condition of anonymity because they were not authorized to speak publicly.

The White House has said that the subsequent meeting with the banker was a pre-inauguration diplomatic encounter, unrelated to business matters. The Russian bank, Vnesheconombank, which has been the subject of U.S. sanctions following Russia’s annexation of Crimea, has said the session was held for business reasons because of Kushner’s role as head of his family’s real estate company. The meeting occurred as Kushner’s company was seeking financing for its troubled $1.8 billion purchase of an office building on Fifth Avenue in New York, and it could raise questions about whether Kushner’s personal financial interests were colliding with his impending role as a public official.

Mueller’s investigation is in a relatively early phase, and it is unclear whether criminal charges will be brought when it is complete.

“We do not know what this report refers to,” Jamie Gorelick, an attorney for Kushner, said in an email. “It would be standard practice for the Special Counsel to examine financial records to look for anything related to ­Russia. Mr. Kushner previously volunteered to share with Congress what he knows about ­Russia-related matters. He will do the same if he is contacted in connection with any other inquiry.”

Kushner has agreed to discuss his Russian contacts with the Senate Intelligence Committee, which is conducting one of several investigations into Russian meddling in the 2016 election.

Kushner rarely speaks publicly about his role in the White House, but he has become a major figure in the administration with a sprawling list of policy responsibilities that includes Canada and Mexico, China, and peace in the Middle East.

Peter Carr, a spokesman for Mueller, declined to comment for this article but said that “the Special Counsel’s Office has undertaken stringent controls to prohibit unauthorized disclosures and will deal severely with any member who engages in this conduct.”

Mueller, who was appointed as special counsel by Deputy Attorney General Rod J. Rosenstein on May 17, is investigating possible coordination between the Kremlin and the Trump campaign to influence the 2016 presidential election and related matters. The inquiry has expanded to include an examination of whether Trump attempted to obstruct justice, The Post reported Wednesday.

Trump on Thursday tweeted that the investigation was “the single greatest WITCH HUNT in American political history — led by some very bad and conflicted people!”

Trump compared his position with the investigation of Hillary Clinton’s use of a private email server in another tweet.

“Crooked H destroyed phones w/ hammer, ‘bleached’ emails, & had husband meet w/AG days before she was cleared — & they talk about obstruction?” he wrote.

After Trump fired FBI Director James B. Comey, Trump said that Comey had told him three times that he was not under investigation. Comey confirmed that in his testimony before the Senate Intelligence Committee last week. The first time he told Trump was in his first meeting with the then-president-elect before the inauguration, on Jan. 6.

Before he met with Trump, Comey gathered his leadership team at the FBI to discuss whether he should be prepared to assure the president-elect that the FBI was not investigating him personally.

Comey testified that not everyone on his team agreed he should. Comey did not name the dissenter, but The Post has learned it was FBI general counsel James A. Baker. Comey testified that the member of his leadership team said that although it was true at the moment that Trump was not under investigation, it was possible that could change.

“His concern was, because we’re looking at the potential — again, that’s the subject of the investigation — coordination between the campaign and Russia, because it was President Trump — President-elect Trump’s campaign, this person’s view was, inevitably, his behavior, his conduct will fall within the scope of that work,” Comey said.

“And so he was reluctant to make the statement that I made,” Comey said.

Baker’s views did not change, even as Comey told Trump a second and third time that he was not being investigated.

“His view was still that it . . . could be misleading, because the nature of the investigation was such that it might well touch — obviously, it would touch the campaign, and the person at the head of the campaign would be the candidate. And so that was his view throughout,” Comey said.

Baker declined to comment.

In the days following Trump’s firing of Comey on May 9 and before Mueller’s appointment, the obstruction-of-justice investigation of the president began, according to people familiar with the matter.

Discussing the firing of Comey, Trump said in an interview with NBC, “In fact, when I decided to just do it, I said to myself, I said, ‘You know, this Russia thing with Trump and Russia is a made-up story. It’s an excuse by the Democrats for having lost an election that they should have won.’”

Comey took notes after each of his nine meetings or phone calls with Trump, including one alone with the president in the Oval Office on Feb. 14, the day after Flynn was forced to resign. Comey testified that Trump said to him, “I hope you can let this go.” The president has denied that he told Comey to drop the Flynn investigation.

Comey told lawmakers he gave his notes to Mueller.

Two senior intelligence officials, Daniel Coats, the director of national intelligence, and Mike Rogers, the head of the National Security Agency, agreed to be interviewed by Mueller as early as this week.

Trump spoke to Coats and Rogers about the Russia investigation, according to officials. Coats told associates that Trump asked him whether he could intervene with Comey to get the FBI to back off its focus on Flynn, the officials said. Coats later told lawmakers he did not feel pressured to intervene.

Trump later telephoned Coats and Rogers to separately ask them to issue public statements denying that there was evidence of coordination between Trump’s campaign and Russian officials. Coats and Rogers refused to comply with the president’s requests, officials said.